Consolidate into a single place the logic to get stats for a given patch
We used to have this logic in three places:
- repo_pull_request.html
- _repo_renderdiff.html
- pagure.api.forks.py
This is now all in pagure.lib.git and exposed in the template via the
``patch_stats`` jinja filter.
Fixes https://pagure.io/pagure/issue/3868
Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>